1. Field of the Invention
The present invention relates to a method of fractionating dispersions of oxidic nanoparticles by membrane filtration. It further relates to dispersions of oxidic nanoparticles that are obtained by the method of the invention.
2. Description of the Background
Recent years have seen a steady increase of interest, from both academia and industry, in nanoscale particles, in other words particles with a diameter of less than 1 μm, since the properties of nanoparticles have caused them to be ascribed great potential in respect of applications in, for example, electronics, optics, and chemical products. Of particular interest in this context are particles whose diameter is in the range below 100 nm. It is here, usually, that the effects known as “nano-effects” occur, quantum effects for example, which can be attributed to factors that include the influence of the large particle surface area. With these particles, moreover, light scattering decreases to such an extent that it is possible to observe increasing transparency in “nano-composites”, in which the particles described are embedded in a matrix, frequently of polymers or coating materials, in order to enhance their properties.
For the application of nanoparticles in composites, however, it is important that the spherical particles first do not agglomerate and second are present in a narrow size distribution. Even small fractions of relatively coarse particles or of agglomerates may adversely affect the properties of the composites. This is true in particular for transparency. Nanoparticles are often adapted to the specific matrix by being modified, the aim of such modification being to produce better dispersion and hence to prevent agglomerates forming.
There are a variety of methods by which the synthesis of nanoparticles can be performed. In addition to gas-phase synthesis, it is possible to operate in solution, and in that case templates are sometimes used. Another option is to grind coarser particles. A feature of this approach is that it is more cost-effective than synthesis from molecular precursors.
Whether from the synthesis of the particles from molecular precursors or from grinding, the resulting product always has a size distribution. Whereas, in the case of particles in the micrometer range, separation of relatively coarse particles can be achieved via sedimentation, centrifugation or screen filtration, these methods are of only limited utility in the case of nanoparticles. If the nanoparticles are in a dispersion, coarser particles may possibly also be separated off via sedimentation or centrifugation, but in this case the extreme surface-area and time requirements, along with the batch operating regime, are so disadvantageous that these methods can in practice be of virtually no importance. Similarly, techniques such as size exclusion chromatography (SEC) or gel electrophoresis are unsuited to relatively large quantities.
For many applications, nanoparticles that are of interest are those composed of metal oxides, as, for example, for the production of UV-protected polymer composites or of fluorescent materials (Journal of Nanoscience and Nanotechnology, 2006, 6, 409-413). For industrial practice, therefore, it would be useful to have a continuous, easily implemented method available for the fractionation of oxidic nanoparticles. There have been a number of proposals to use membrane filtration methods for this purpose. It should be noted here that separation on a membrane is influenced by the specific interaction between particle and membrane.
In order, generally, to separate particles from suspensions according to specified criteria such as particle diameter, for example, it is common to employ filtration techniques. In that case, dead-end filtrations, as a batch operation, or crossflow filtrations, as a dynamic operation, are generally used. In the case of dead-end filtration, the entire volume to be filtered is passed directly through the filtration medium, and in this case the particles deposited are generally able to build up a cake, which in turn critically co-determines the outcome of the filtration. The filtration outcome here, then, is determined not only by the properties of the filtration medium but also, in particular, by the filter cake which has formed and which changes during the operating time. Cake-forming filtration, accordingly, cannot be used to classify particle dispersions. Only depth filtration, which operates in accordance with the dead-end method, is able, within certain limits, to effect classification, by virtue of the fact that the particles to be separated penetrate the structure of the filter medium and are separated on the basis of their adhesion to the internal surface area of the filter medium. The limits on this method are that only very dilute dispersions can be treated and that the classifying effect has a high inherent imprecision; as a result of this, significant amounts of the target product remain adhering in the filter medium and are therefore lost.
In the case of crossflow filtration, the medium to be filtered is conveyed tangentially over the filtration medium. It is the pore size of the filtration medium that determines the cut-off limit. Important applications are in microfiltration, ultrafiltration and nanofiltration.
Crossflow filtration attempts to circumvent the disadvantages of the dead-end filtration method by virtue of the fact that, in this case, in contrast to the conventional filtration, the flow impinging on the filter medium is tangential. The feed stream is divided into a filtration stream through the filter medium and the flow over and parallel to the filter medium. In membrane technology, the flow which passes through the membrane is termed the permeate. The material retained on the membrane is termed the retentate. As a result of this flow regime, the retained component is transported back from the surface of the filtration medium into the retentate flow. Hence this counteracts the formation of deposits and cake layers on the filtration medium.
Advanced Materials 2005, 17 (5), 532-535 describes how the technique of membrane crossflow filtration can be utilized for fractionating metallic nanoparticles. For that purpose a special membrane is produced that contains nanoscale channels. The experiments were conducted on the very small laboratory scale, and do not offer any indications of transfer to the industrial scale. Nothing meaningful is said about either the pore radius or the composition of the membranes necessary for fractionating oxidic nanoparticles. Moreover, it is observed that classification of nanoparticles is not possible with conventional membranes.
Journal of Membrane Science 2006, 284, 361-372 describes the crossflow membrane filtration of a dispersion of silicon dioxide nanoparticles. However, no details are given here of the possibility of fractionation; instead, the investigation was of the formation from the nanoparticles of a dynamic cake layer, which renders fractionation impossible.
The firm Bokela, Karlsruhe (Germany) markets a screen filtration system (Dynofilter) which allows coarse fractions of down to 10 μm to be separated from a particle dispersion by dynamic screen filtration. As far as the possibility of using membranes in this system is concerned, nothing is known.
Langmuir 1997, 13, 1820-1826 describes investigations of the membrane filtration of polymer particles with a permanent surface charge. There, the retention of nanoparticles is improved by deliberate introduction of surface charges. As far as classifying effects are concerned, nothing is stated.
Anal. Chem. 2006, 78, 8105-8112 outlines how organic colloids are separated from aqueous solution by crossflow ultrafiltration. The membrane there retains more than 99% of nanoparticles.
US 2004/0067485 A1 describes the synthesis of nanoscale semiconductors based on zinc and cadmium, combined with the elements S, Se and Te, using a protein as a template. It is indicated that the resulting complex of, for example, zinc sulphide/apoferritin may be able to be fractionated by dead-end membrane filtration, the diameter of the pores of the membrane used being significantly higher than the diameter of the particles. A large selection of membrane materials is cited, but with no examples for the filtration method. With this method it is not possible to carry out classifying nanoparticle filtration on an industrial scale, since a filter cake would be formed. Consequently, the proposed method remains able to be carried out on a laboratory scale only, where the frequent change of filter required is easily possible.
WO 2006/116798 A1 describes the production of radioactive nanoparticles based on metallic technetium, these particles undergoing a dead-end membrane filtration procedure. The membrane used in that procedure is hydrophilic. Here as well, therefore, the dead-end method is used, which even with a low level of nanoparticle agglomeration results in virtually complete deposition of all the particles on the membrane. The method proposed there for nanoparticle fractionation can therefore be carried out only at very low nanoparticle concentrations and on a laboratory scale.
In the prior art there is no known membrane method which can be used on an industrial scale and in which a fine fraction can be classified out of a particle dispersion which contains predominantly nanoparticles but also coarser particles. All of the prior art methods form a cake layer on the membrane surface, and so, although separation is made possible, the classification of nanoparticles from a dispersion is not.